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Change the width of a chart plot with different number of bins

I have the following chart (code is at the end)

enter image description here

and I need to increase its width.

If I set alt.Chart(df, width=100, height=90) then the bar width of the group with fewer bars is too large, like this

enter image description here

If I set alt.Chart(df, height=90).mark_bar(size=20) then bars overlap, like this

enter image description here

import numpy as np
import pandas as pd
import altair as alt

np.random.seed(0)

model_keys = ['M1', 'M2']
scene_keys = ['S1', 'S2', 'S3']
layer_keys = ['L1', 'L2']

ys = []
models = []
dataset = []
layers = []
scenes = []

for sc in scene_keys:
    for m in model_keys:
        for l in layer_keys:
            if sc == 'S3' and l == 'L1':
                continue
            for s in range(10):
                y = np.random.rand(10) / 10
                data_y = list(y)
                ys += data_y
                scenes += [sc] * len(data_y)
                models += [m] * len(data_y)
                layers += [l] * len(data_y)


df = pd.DataFrame({'Y': ys,
                   'Model': models,
                   'Layer': layers,
                   'Scenes': scenes})

bars = alt.Chart(df, height=90).mark_bar().encode(
    x=alt.X('Scenes:N',
        title=None,
        axis=alt.Axis(
            grid=False,
            title=None,
            labels=False,
        ),
    ),
    y=alt.Y('Y:Q',
        aggregate='mean',
        axis=alt.Axis(
            grid=True,
            title='Y',
            titleFontWeight='normal',
        ),
    ),
)

bars = bars.facet(
    row=alt.Row('Model:N',
        title=None,
    ),
    column=alt.Column('Layer:N',
        title=None,
    ),
    spacing={"row": 10, "column": 10},
)

bars = bars.resolve_scale(x='independent')

bars.save('test.html')
Solution

  • Rather than specifying the width of the bars (which does not affect the default width of the chart), you can specify the chart width via the step property, which will adjust both the bar width and the chart width together:

    alt.Chart(df, height=90, width={"step": 20}).mark_bar().encode(...